Selectivity in Nerve Cells

In contrast to other cells, a nerve cell or neuron possesses sections known as dendrites and axons. A dendrite consists of a large number of short protrusions like roots or branches and plays a role in receiving stimuli from other neurons and cells and transmitting them to the cell body. Axons are long, thin fibers emerging from the body of the cell, consisting of one single part along which stimuli are sent, serving to transmit messages to the brain. Nerve cells thus form a dense network of long chains.

Cells are simply too complex to have evolved randomly; intelligence was required to produce them —Michael Behe

On the left you can see the general structure of a nerve cell (neuron). This structure of nerve cells is specially created to transmit signals. Because of these structures, you perceive what is happening around you and act as you wish.

Every cell has an electrical charge around its membrane. Every neuron resembles a miniatured biological battery ready to discharge its energy. The ions, electrically charged molecules both inside and outside every nerve cell, set up a difference of electrical charges along the length of the cell membrane. To send a stimulus, human neurons require an average electrical charge of minus 50 millivolts (One millivolt is 1 thousandth of a volt).50 At this point, the nerve signal is transmitted by the axon. After every nerve signal passes, potassium ions flow from the cell membrane. After every signal, the neuron must be recharged. In order to do that, it takes ions back until the potential level is regained.

1. Myelin sheath
2. Axon
3. Signals moving my jumping from one synapse to another speed up the pace of transmission.

Electrical current accumulated on a myelin sheathed axon ganglion continues on by moving from one to the next. Signals are thus transmitted along the nerve cells. Every detail of this system has been specially created for you to perceive the outside world and react as necessary, and to establish coordination with the brain for your body to perform all its vital functions.

For a neuron to send a signal takes around 1 thousandth of a second, so in one second, it is possible to send at most 1,000 signals. In general, however, only 300 to 400 signals take place every second. Human nerve cells are of different lengths.51 The transmission between cells takes place at speeds between 3 and 100 meters (9.8 and 328 feet) per second. 52

Professor Peter Suckling, a neurophysicist at the Downstate Medical Center, speaks of the cell membrane with great amazement: "This very thin membrane can sustain an electrical tension better than most insulators. The insulation strength is high. It has to be strong; it's so very thin." 53

Nerve cells are able to communicate as a result of the electricity produced in the cell membrane, to transmit information from one place to another, and ensure the healthy operation of the body's functions. In addition, these electrical messages produced in the cell reach their intended destinations, carrying a message for the recipient cell. Every cell knows the meaning of the message that reaches it, and begins operating accordingly. Were there no such flawlessly operating systems among the cells, it would be impossible for any living thing to maintain vital functions. That being so, then how did such a flawless system requiring consciousness and intelligence arise? It is of course impossible to maintain that unconscious masses of atoms and molecules took a decision to form cells and that such a system later spontaneously came into being among those cells. The existence of such a conscious system proves that living things were created. The magnificent creation in such microscopic dimensions that astonishes scientists belongs to our Lord, the Creator of all.

"Is He Who creates like him who does not create? So will you not pay heed?" (Surat an-Nahl, 17)

Movement potential forms through the action of the closed voltage sodium and potassium channels in the axon membrane. At rest, these channels are generally closed, and open when the cell membrane potential attains a negative value. Potassium channels open in a millisecond, and sodium channels in even less time. It has become possible to identify and follow all these processes through 20th-century technology.

The Neuron at Rest

When a nerve cell is not transmitting a signal, it is at rest, but is not entirely immobile. It must be ready to forward messages that may arrive at any moment from neighboring cells. A neuron at rest must always be polarized, so that the fluid within it is more negatively charged than the fluid outside. Along its membrane, a nerve cell has an electrical potential of approximately 70 millivolts. This is called the membrane potential or resting potential. Though this appears very small, it means that a tiny cell produces up to 1/20th of the voltage of a flashlight battery, and the axon holds the potential for electrical activity along the membrane. How does this resting potential come about? How is it conserved?

Outside the axon, there are sodium (Na+) and chloride (Cl-) ions. Inside are charged proteins and potassium (K+) ions. The electrical imbalance between the cell membrane and the exterior forms a resting potential along the membrane, an imbalance formed by the charged ions that lets the cell membrane be selectively permeable for different ions. Sodium, potassium and chloride ions pass through the cell membrane, but the passage of highly-charged proteins is restricted—and thus, so is the formation of electrical potential.

However, selective permeability cannot be the sole answer, because the number of potassium (K+) ions in the cell is always greater than the number of sodium (Na+) ions. Furthermore there are more sodium (Na+) ions outside the cell than there are potassium (K+) ions. For the requisite ion balance to be established, the densities inside the nerve cell must be reversed.

The cell achieves this by using the kind of ion pump we touched on earlier. The sodium-potassium pump, a protein molecule, forms a channel in the cell membrane, taking its energy from ATP (or adenosine triphosphate, the cellular energy molecule used directly by living things) and takes in potassium (K+) ions as it expels sodium (Na+) ions. In this way, the correct ion proportions are maintained inside and outside the cell. There are between 100 and 200 sodium-potassium pumps on every square micrometer on the cell membrane surface. Each one expels 200 sodium ions a second, and admits 130 potassium ions.

Movement Potential and Signal Transportation

The signal begins when a neuron is stimulated by another neuron or its environment. Immediately afterward, the signal acts along the length of the axon, causing the cell membrane's potential to suddenly reverse. In the cell membrane, there are thousands of protein channels or gates for the passage of ions, but these gates are generally closed. In the event of a signal, the sodium channels open, and positively charged sodium ions flow in. Temporarily, therefore, the interior of the cell membrane has a greater positive charge than the outside, and the resting potential is reversed, raising the cell membrane's potential to +50 millivolts. The reversal of these charges is called movement potential. During movement potential, the potassium gates open, and positively charged potassium ions flow out. This again balances the resting potential, so that the neuron's interior is again negatively charged and the exterior positively charged.

One single nerve impulse triggers this entire process. You can therefore compare the signals to dominos. As one domino falls over, it pushes the one next to it. But in this case, as the signal passes, the "dominos" right themselves again and stand up, preparing themselves for a subsequent movement potential.

How Much Do You Know About What Takes Place in Your Cells, with all Their Responsibilities?

(A) RESTING POTENTIAL

1. Axon
2. Electrodes

3. The sodium-potassium pump transports Na+ ions out of the cell, and K+ ions inside it, against the pressure of concentration. This process requires ATP.

4. Na+ ion gates are active gates with a voltage that permits Na+ ions to pass swiftly into the cell. Ions are absorbed during a motion potential in the light of the concentration slope. Na+ ion gates are closed during resting potential.

5. K+ ion gates are active gates with a voltage that enables K+ ions to be absorbed outside the cell when they are opened. K+ ion gates are closed during resting potential.

6. Negatively charged protein ions cannot be absorbed, since they impart a negative charge to the inner part of the neuron.

(B) MOVEMENT POTENTIAL

Stage 1: Disruption of polarity

1. Na+ ions move in at high speed. Na+ ion gates open in reaction to a stimulus. Na+ rapidly enters the cell by disrupting polarity. The inner part of the axon rises from -70 mV to +30 mV.

(C) MOVEMENT POTENTIAL:

Stage 2: Repolarization

1. K+ ions rapidly move out. K+ ion gates open shortly after, and K+ ions begin moving out of the cell at high speed. The cell is thus repolarized, falling to under -70 mV.

Many people feel no need for the information in this book, unless they are particularly interested or it falls within their sphere of expertise. That is because this information will make no change or difference in their lives. Yet our cells have vital responsibilities, implementing them without ever making a mistake. Cells carry out all these activities ceaselessly, without ever making you aware of them. They remain devoted to these tasks for decades, never going wrong, forgetting or needing rest. The signals carried along channels that open and close in less than a second are actually invisible portions of an enormous Creation. As a result of this system, you can taste the apple you eat, smell the scent of a rose, recognize your mother's voice, remember a telephone number, can operate your television's remote control, answer a question, run down stairs, and do all these things without ever thinking about them. Your responsibility is to be grateful to our Lord Who created this system in your body without your ever realizing you needed it.

- At the beginning of movement potential, most sodium and potassium channels are closed.

- Electrical current passing by the axon or carrying a stimulus causes the potential of the axon's cell membrane to fall.

- This opens some of the sodium channels, and sodium ions flow into the cell.

- Though positive charges enter in this way, the cell membrane potential is still negatively charged. Therefore, sodium channels open, and more sodium ions enter.

- At this point, the exact opposite happens to the axon: Sodium channels begin closing one after the other, and the inward flow of sodium is halted.

- Finally, as the positive charge inside the cell declines, the cell membrane potential returns to its former level.

Synapse Paths

The human nervous system is a complex network consisting of billions of nerve cells that establish communication among themselves and other cells in the body by means of synapses—small parts of the nerve cells that approach one another very closely but never actually touch. Since they never come into contact with one another, signals do not pass from one cell to another directly, but are carried through the gaps by chemical neurotransmitters.

When a signal reaches the transmitting cell, that cell causes some neurotransmitters to be secreted into the extracellular space. At this the neurotransmitter molecules diffuse in this space—passing directly into a less dense environment—and attach to the receptor protein molecules on the second cell. Since there are many kinds of neurotransmitter and receptor molecules, the synapse can cross very quickly (in 1/1000th of a second) or rather slowly (1/100th of a second). Chemical agents set the second cell in action, or else halt it. Therefore, synapses serve to alter the information in the nervous system or set it in motion. Because of these properties, the function of synapses in the brain is connected with learning and memory.

As neurons transmit messages by means of the synapses, they exchange chemical signals. The nerve cells in your brain have 100 trillion connections, where there is a constant and enormous molecular traffic. The electrically-charged chemicals known as ions and large and small varieties of proteins tell this traffic when to flow, and when to halt.

Considering The Extraordinary Activities in our Bodies is an Opportunity to Properly Appreciate Our Lord.

Muscle Movement and Acetylcholine Channels:

A muscle contracts when the motor nerve it's attached to is stimulated. When it reaches the terminals, the nerve transmission in the axon belonging to this motor nerve enables a transmitter known as acetylcholine to be secreted. This substance spreads in the space known as the synapse between the nerve and muscle cell, binding to complex proteins (acetylcholine receptors) in the muscle cell membrane.

This binding causes the ion channel in every receptor to open, and thus the current continues along the muscle cell membrane. This causes electrical activity in the muscle cell, resulting in the muscle's contraction. By means of the perfect functioning of these systems, you are able to turn the pages of this book with ease, sip tea with one hand and tap a rhythm out with the fingers of the other.

One way of stopping this chain of events permitting muscle movement is to block the acetylcholine receptors. This method is employed by some poisonous animals to cause paralysis. The venom of the black widow spider, for instance, causes acetylcholine disruption.

As you have seen, living your life in comfort, doing what you like when you like, perceiving the world around you and reacting appropriately, all take place flawlessly by means of this special system Allah has created in your body. However, acetylcholine channels represent but a tiny part of your body's complex Creation. Yet every detail in the body has been created for some important function with the knowledge of our Lord.

Synapses work like biological keys to transmit electro-chemical signals. Every synapse consists of two parts: the rounded part at the end of the transmitter neuron, and the receiving region in the target cell membrane. The synaptic gap is as tiny as 1 billionth of an inch (2.54 centimeters) long and separates the two cell membranes.

At the bulb-shaped end of the synapse are very small spheres called synaptic sacs. Each one contains thousands of molecules that transmit stimuli.

When the nerve signal reaches the tip, the sacs attach to the membrane and release their contents into the synaptic cavity. This enables specific receptor channels to open and sodium ions to move rapidly towards the target cell. This ion flow sets one region of the target cell membrane into operation, and an electrical impulse is formed within the target cell.

This molecular traffic among our nerve cells, here described in very superficial terms, continues constantly at very high levels. Ions and some proteins determine when this traffic should stop or start. It is of course impossible for molecules to construct a nervous system of their own accord and subsequently to organize it in a most impeccable manner. These molecules have come together within a specific Creation, to serve a particular purpose. This system is one of the examples exhibiting Allah's infinite dominion.

Special Creation in The Cell Membrane's Chloride Channel

At the side you can see how the chloride channel structure is organized and the way in which salt is absorbed into your cells. Two equal sub-units, colored blue and green, are seen in the channel. Each sub-unit constitutes its own ion space. The red spheres show the pathways of chloride ions.

Nature The 17 January, 2002 edition of Nature magazine carried three-dimensional images of the chloride ion channel, obtained by using X-ray crystallography. Roderick MacKinnon, a Howard Hughes Institute researcher at Rockefeller University, and his team revealed the protein architecture specially created for chloride ions to pass through the cell membrane in the most efficient manner.1 About the complex structure they encountered, MacKinnon said:

It is a complicated structure. "Scientists did an excellent job deducing many aspects of the chloride ion channel . . . To understand the physical principles of anion selectivity, an atomic structure is needed. Although the structure is complicated, it conveys a simple message of how nature arranged the protein to stabilize an anion such as chloride inside the membrane. 2

Living things use electrically charged ions for communications of various kinds—control of the heart rhythm, the formation of nerve signals and the secretion of hormones. As explained earlier, cells use ions to transmit signals by setting up an electrical differential between the inside and the outside of the cell. When ions are charged, they prefer to be in water instead of in a membrane composed of fat. The ion channels, able to distinguish two ions, offer a solution to that problem.

In human beings, there are nine different chloride channels that serve different functions, from salt absorption in the kidneys to muscle contraction. The chloride ion channel possesses a totally different structure from that of the potassium ion channel. Meanwhile, the potassium ion channel is largely filled with water, of a pyramidal shape. The chloride channel has two spaces, each with a narrower space in the middle, rather like an hourglass or egg-timer. Scientists have found that the protein sub-units comprising both channels are laid out in entirely different ways. In the potassium channel, four protein sub-units constitute a single space. In the chloride ion channel, each protein sub-unit has its own space and both halves of the sub-units possess opposite faces in a way known as two-layered reversible symmetry.

Understanding this structure may help scientists understand how the channel opens and closes to maintain the correct ion concentration. As you saw, despite all their advanced technology, scientists have still not fully unraveled the complex processes that transpire within their own bodies, within the cell membrane. R. MacKinnon, who studies the ion channels, has stated that these structures—the gateway to the cells—have only newly begun to be understood, and that continuing experiments seek to understand how they function.3

The cell membrane possesses a special structure for admitting chloride ions. Despite all obstacles, the cell membrane is capable of admitting the necessary ions. That exceptional solution cannot, of course, be the product of molecules devoid of intellect and consciousness. The order here is a part of the complex system that Allah has created in our cells.

3. Brain Cell Selectivity: "The Blood-Brain Barrier"

The picture above shows absorption of alcohol molecules by brain cell membranes. Alcohol molecules close brain cells down one by one by preventing the passage of nerve signals.

In the brain, special sentries admit the necessary nutrients in the blood but keep out substances that would keep nerve cells from functioning. These sentries form a barrier between the brain's nerve tissues and prevent substances in the blood from entering. This barrier is constituted by endothelium cells that line the brain's blood vessels. The barrier's importance stems from the fact that nerve cells need a specific chemical environment. Lacking such an obstacle, when you ate foods that increase the concentrations of glucose, amino acids, hormones or other compounds, or when you engaged in exercise, nerve functions would move out of control and would even suffer seizures.

In the brain, countless capillaries bring nutrients and carry away waste products. The brain's endothelium cells have special connections that prevent substances in the blood from passing through the cell membrane and reaching the nerve tissue. For that reason, the endothelium cells are almost totally impermeable. But did the barrier not allow anything pass, then the brain would be deprived of the needed oxygen, glucose and amino acids and would die. Yet the blood-brain barrier's special mechanisms keep unwanted substances out, but carry vital molecules to the brain.

Molecules soluble in fat can generally pass through the blood-brain barrier immediately. These include nicotine, ethanol and heroin. However, if charged molecules are not soluble in fat and must depend on special transport systems, they enter the brain very slowly, or not at all. These may be large molecules such as proteins or small ones like sodium. Glucose—the brain's main source of the energy it needs—and the amino acids that it cannot produce itself, are not soluble in fat. Therefore, these substances are carried through the cell membrane by particular transporters. The human brain uses more than 120 grams of glucose a day. But since it cannot store more than 2 grams, glucose must be constantly pass through the barrier.

In the light of this need, a great many transporters in every endothelium cell allow it to absorb large quantities of glucose from the blood. The glucose transport system is the body's hardest-working transport system. The cell itself uses only a very small part of this sugar, and the remainder is transferred to the brain. Yet the structure of the transport molecules is still a mystery to scientists. In all likelihood, the transporters are one or more proteins that open the channels so as to permit glucose to pass through the cell membrane.

Amino acid transport systems are far more complex, because every one of the 20 amino acids has a different molecular structure. These can be grouped into four classes depending on their chemical properties: large neutral, small neutral, basic and acidic. Each category has its own transport system. As with glucose transporters, large neutral amino acid transporters lie on both sides of the barrier, and amino acids can thus enter and leave the brain. Small neutral amino acids can be synthesized by the brain cells, so there is no need for them to be transported to the brain.

The idea of the blood-brain barrier was first advanced by the German bacteriologist Paul Ehrlich at the end of the 19th century. It was possible to be proved, however, only with the development of the electron microscope in the 1950s. Although in appearance capillaries in the brain resemble the veins in other parts of the body, they possess different features. First of all, the connections between cells in the capillary vein in the brain are exceedingly dense. At their connection points, cell membranes are attached to one another just like a zipper. Unions between the endothelium cells in the capillary vessels in the other parts of the body have spaces. Second, in cells in the brain's capillary vessels, there are very few pinocytosis sacs that help transport fluids and solutes through the cell membrane. In cells outside the brain, however, these sacs are widespread.

It is Allah Who made the Earth a stable home for you and the sky a dome, and formed you, giving you the best of forms,and provided you with good and wholesome things. That is Allah, your Lord. Blessed be Allah, the Lord of all the worlds.
(Surah Ghafir, 64)

The importance of the barrier

We can better understand the importance of the blood-brain barrier from diseases that occur in that barrier's absence. Tumors, brain-tissue defects and edema—swellings due to the accumulation of fluids and proteins—cause this barrier to collapse. Since fewer sacs form in the walls of the endothelium cells, leakage begins, or the tight bonds between the cells are loosened.

Damage to the barrier leads to the accumulation of fluid and lead poisoning in brain tissue. The metal first enters the endothelium cells and then the astrocytes. After the lead has damaged the barrier, the brain is more vulnerable to other substances.

The blood-brain barrier is no longer considered a passive structure, but a dynamic interface between the blood and the brain. Yet our understanding of its maintenance and transport mechanisms is still incomplete. 54

Every portion of our bodies has been specially created for life. These parts, only a few of which we shall look at in this book, have occupied scientists for decades and contain mechanisms that astound investigators. Why is this blood-brain barrier in exactly the right place only and not among cells in any other part of the body? How do the cells know that the brain needs a stable environment, and so entry to and exit from its cells must be more tightly controlled? No doubt the cells themselves do not decide to form a barrier and then construct one in the membranes. It is out of the question for this to be the work of coincidence. The barrier in the brain serves a vital purpose, and this complex structure is directed towards that end.

Therefore, Darwinists hoping to account for the origin of life in terms of coincidental mechanisms are once again at a dead end. Even if all the body's complex systems existed but only this barrier in the brain cells did not, then you could not survive. That being so this same barrier must have been present, along with all its systems, from the very outset. Gradual development, the basis of the Darwinists' claims, is again invalidated, as this example shows.

As an indicator of planned structure, this precautionary measure taken for human beings is one of the countless proofs of the existence of Allah.

"Then inquire of them: Is it they who are stronger in structure or other things We have created? We created them from sticky clay. No wonder you are surprised as they laugh with scorn!" (Surat as-Saffat, 11-12)

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